Vehicles commonly include an automatic transmission that is driven by a powerplant. Powerplants produce drive torque that is transferred to the automatic transmission and can include, but are not limited to, internal combustion engines, electric machines or combinations thereof (e.g., hybrid powerplant). The automatic transmission transfers the drive torque to a driveline at varying gear ratios to propel the vehicle.
The automatic transmission includes a plurality of gear sets and clutches. The clutches are hydraulically actuated to selectively engage the gear sets to provide a desired gear ratio. Pressurized hydraulic fluid is provided by a pump. Typically, the pump is driven by the powerplant and compresses the hydraulic fluid drawn in from a sump.
When a vehicle sits for an extended period of time (i.e., soak), the hydraulic fluid completely drains into the sump. In colder ambient temperatures (e.g., 30° F. and lower), the hydraulic fluid is cooled and becomes more viscous. When the powerplant is initially cranked after an extended soak in a colder ambient environment, the pump can cavitate as a result of the highly viscous hydraulic fluid. The cavitation generates a rattle or noise that can be sensed by the vehicle operator, which is undesirable.